1. Field of the Invention
The invention relates to a circuit board on which a plurality of heat generating elements have been formed and a manufacturing method of such a circuit board. More particularly, the invention relates to a circuit board for a liquid discharging apparatus which converts an electric energy into a thermal energy by heat generating elements and discharges a liquid by using the thermal energy and a manufacturing method of such a circuit board.
2. Related Background Art
A manufacturing method of a conventional circuit board will be described hereinbelow with respect to an ink jet head as an example.
In an ink jet recording apparatus, ink is discharged as a micro liquid droplet from a discharge port onto a material to be recorded, so that an image can be recorded. Its principle will now be described. An electric energy is converted into a thermal energy by a heat generating element and a bubble is generated in the ink by the thermal energy. The liquid droplet is discharged from the discharge port existing in a front edge portion of a liquid discharge head by the operation of the bubble and deposited onto the material to be recorded, so that the image is recorded. Therefore, such a liquid discharge head has a circuit board on which a plurality of heat generating elements each for converting the electric energy into the thermal energy have been formed.
Specifically, after a resistor layer and an electrode material layer were formed onto an insulative surface, a part of the electrode material layer is removed, a pair of electrodes are formed, and an interval between the electrodes becomes a heat generating portion. After that, a protective layer to protect them against the ink and a cavitation resistive film to protect the protective layer against a chemical or physical damage accompanied by heat generation are formed.
In such a construction, there is a case where a step coverage of edge portions of the electrodes to form the heat generating portion deteriorates. It is schematically shown in FIG. 7. A resistor layer 62 of TaSiN or the like is formed on an oxide layer 61 of SiO2 on an Si substrate. Although a wiring layer 63 of, for example, Al is formed on the resistor layer 62, a portion without Al exists on a part of the resistor layer 62. The portion without Al is a heat generating portion 64 of the heat generating element. A protective layer 65 consisting of P-SiN (SiN film formed by a plasma CVD) or the like to protect those layers against the ink and a cavitation resistive film 66 consisting of Ta or the like to protect the protective layer 65 against the chemical or physical damage accompanied by the heat generation are formed on those layers.
The circuit board for the liquid discharging apparatus has a plurality of heat generating elements as mentioned above at a high density, thereby enabling an image to be recorded. Each heat generating element is serially connected to power transistors (not shown) for on/off controlling a current flowing in the heat generating element. Discharge ports are formed on the circuit board and become the liquid discharging apparatus.
To solve the above problems, as disclosed in Japanese Patent Application Laid -Open Nos. H4-320849 or H4-320850, a method of providing a taper shape for edge portions of a pair of electrodes which the heat generating element has has been developed.
According to such a method, even in the edge portions of a pair of electrodes which the heat generating element has, coating performance of the protective layer and the cavitation resistive film can be improved. A manufacturing method of the circuit board including a forming method of the taper shape portions of the pair of electrodes which the heat generating element has will be described hereinafter.
FIG. 6 shows a flow for a manufacturing process of the circuit board. FIG. 5A shows a cross sectional view of peripheral portions of a heat generating element of the manufactured circuit board. First, an Si wafer is used as a substrate and an oxide layer 41 of SiO2 having a thickness of about a few μm is formed on the Si wafer by a thermal oxidation process. A resistor material layer of TaSiN or the like having a thickness of about 50 nm is formed on the oxide layer 41 by sputtering. After that, for example, an Al film having a thickness of about 200 nm is formed and an electrode material layer is formed. A resist layer (I) is formed on the electrode material layer and patterned. Thereafter, Al and the resistor material layer are dry-etched by using an RIE or the like, thereby separating the element and forming a wiring 43 and a resistor layer 42. After the resist layer (I) was removed by O2 ashing, a resist layer (II) is formed and patterned. Thereafter, Al of a portion serving as a resistor portion 44 of the heat generating element is removed by wet etching. At this time, by using an organic alkali etchant comprising a tetramethylammonium hydrooxide (hereinafter, abbreviated to TMAH) as a main component or an acid etchant comprising a phosphoric acid as a main component as a wet etchant, an edge portion of the resist layer (II) is etched and while it is moved backward, Al is etched. Thus, edge portions of a pair of electrodes which are formed become a taper shape. Subsequently, a protective layer 45 consisting of SiN and having a thickness of about 300 nm is formed by a plasma CVD method. A Ta film 46 is formed by sputtering. By removing unnecessary portions of the Ta film 46 by dry etching, a circuit board is obtained.
However, in recent years, the realization of high fineness of printing has been further progressed and a discharge quantity of the ink per discharging operation is decreasing from tens of pl to a few pl. The realization of a high speed of printing is also strongly demanded. To reduce the reciprocating motion of the liquid discharge head, a long circuit board has been realized. Therefore, the number of heat generating elements per circuit board has been increased from hundreds to thousands. In such a situation, also in the forming method of the pair of electrodes in the foregoing taper shape, such a problem that a slight fluctuation occurs in the taper shape and the coating performance of the protective layer and the cavitation resistive film on a part of the heat generating elements deteriorates occurs.
It is, therefore, an object of the invention to provide a circuit board for a liquid discharging apparatus in which coating performance of a protective layer and a cavitation resistive film on a heat generating element is excellent and durability is excellent and to provide a manufacturing method of such a circuit board.